I have a pair of Z23's ready for the rears once I decide on rotors.I could answer it all, but the way the Z23 pads work and wear i cannot suggest anything else.
Lots of shit changes over from model to model, its one thing Mazda does right.
I've got USR rotors and greenstuff pads in the front that I put on earlier this year but have warped them already and need something better suited for my driving style.
My only holdup on the CX-9 caliper swap is that I don't want to buy calipers, and I already have 2 sets of front calipers with rebuild kits.
But if someone wants to get some new front calipers for the 6, powder coated in black or blue, I'd happily sell them and buy the CX-9 calipers...lol